The present invention relates to semiconductor optical elements and its application to laser diodes which operate with net optical gain or net refractive index kept almost constant even if the other parameter is varied.
Change of injected carrier density, optical gain, and refractive index are related to each other in a semiconductor medium. This results in chirping under a high speed modulation or unwanted amplitude modulation even if pure frequency modulation is desired by changing the injection currents in conventional semiconductor laser diodes. These phenomena degrade transmission performance in optical fiber communication systems, which limit the repeater span due to dispersion or noise. In order to avoid these problems and realize ideal AM (amplitude modulation) or FM (frequency modulation) lasers, external modulators are usually employed. These schemes, however, have drawbacks such as rather complicated device structure, larger size, as well as the limits on the achievable modulation index of FM lasers.
As for a wavelength tunable filter in which a corrugation grating is formed along an active layer and the Bragg wavelength is varied by changing the refractive index associated with carrier injection in the active layer, this carrier change is also accompanied by a gain change leading to changes of transmission and reflection efficiencies at the Bragg wavelength. This structure requires a gain adjusting region to keep the insertion loss constant.